HYDROZOA, ZOOPHYTES.
ZOOPHYTES are animals of a much higher organization than the Protozoa,
inasmuch as they are furnished with special organs of prehension,
offence and defence, of attachment, and in many of locomotion. For the
most part they consist of numerous individuals called Polypes, united in
a community, and living together in intimate sympathy and combined
action, so as to form one single compound animal.
Zoophytes are divided into two groups, namely the Hydrozoa, whose type
is the common fresh-water Hydra, and the Actinozoa, which are composite
animals, including the reef-building corals, whose polypes are formed
according to the type of the Actinia, or common Sea Anemone. The
Hydrozoa consist of seven orders, the first of which are the Hydridæ,
inhabitants of fresh water; the next constitute the oceanic Hydrozoa,
some of which, though extremely varied in form, are connected by the
most wonderful relations.
The solitary Hydra that lives in fresh-water pools and ditches, consists
of a soft cylindrical muscular bag, capable of being stretched into a
slender tube, shrunk into a minute globe, or widely distended at will.
At one end there is a circular mouth, which is highly sensitive,
opening, closing, or protruding like a cone, and surrounded at its base
by six long flexible arms called tentacles, arranged symmetrically. The
mouth opens into a cavity extending throughout the length of the body,
which is the stomach; the other end of the sac is narrow, and terminates
in a disk-shaped sucker, by which the Hydra fixes itself to aquatic
plants, or floating objects, from whence it hangs down, and the
tentacles float in the water.
[Illustration: Fig. 109. Thread-cells and darts.—A, B, C, D,
Thread-cells at rest; E, F, G, H, appearance of the darts when
projected.]
The sac or body is formed of two layers, an inner and an outer layer, of
firmer texture, formed of cells imbedded in a kind of sarcode, and the
space between the two layers is filled with a semifluid substance, mixed
with solid particles and full of vacuoles. The inner and outer layers
are united at the mouth, and the tentacles are closed tubes in
communication with the cavity of the stomach. The exterior layer of the
tentacles is beset with wart-like excrescences, formed of clusters of
cells, with a larger one in the centre filled with a liquid. In all of
them a long spicula, or sting, often serrated at the edge, is coiled up
like a thread, and fixed by one end to a kind of tube, like the inverted
finger of a glove, that the animal can dart out in an instant.
Thus armed, the tentacles are formidable weapons; they are highly
contractile and wonderfully strong, tenaciously adhering to the small
worms and aquatic insects on which the Hydræ feed, and they are aided by
the roughness of their surface. They transfix their prey, and are
believed to infuse a liquid poison from the dart, or thread-cells, into
the wound, then twisting their other tentacles round the victim, it is
instantly conveyed to the mouth, and slowly forced into the digesting
cavity, where it is seen through the transparent skin to move for a
short time, but as soon as the nutritious juice is extracted, the animal
ejects the refuse by its mouth. In the inner layer, enclosing the cavity
of the stomach, there are cells containing a clear liquid with coloured
particles floating in it, which is supposed to perform the part of a
liver; and, as the Hydræ have no respiratory organs, their juices are
aërated through their skin. They have no perceptible nerves nor nerve
centres, yet they are irritable, eminently contractile, and are
attracted towards the light—all these being probably sympathetic
motions.
Though in general stationary, the Hydra can change its place; it bends
its body, stretches to a little distance, and fixes its anterior
extremity firmly by its tentacles; then it detaches its sucker and
brings it close to its mouth, fixes it, and again stretches its fore
part to a little distance along its path, and repeats the same process,
so that it moves exactly after the manner of certain caterpillars. It
can even move along the water by attaching the expanded disk of its
sucker to the surface, where it soon dries on being exposed to the air,
and becomes a float, from whence the Hydra hangs down with its tentacles
extended like fishing lines, as in fig. 110; or it can use them as oars
to row itself along under the surface of the water.
On account of their simple organization, the Hydræ are endowed with the
most astonishing tenacity of life. As the whole animal is nourished from
the surface of the digestive cavity, they appear to suffer no
inconvenience from being turned inside-out, the new cavity performing
all the functions of digestion as well as the old one. They may be cut
into any number of pieces, and, after a little time, each piece becomes
a perfect Hydra. The head may be cut off and they get a new one; or it
may be split into two or three parts or more, and the animal becomes
many-headed; and, what is still more marvellous, two Hydræ may be
grafted together direct, or head and tail, and they combine into one
animal.
[Illustration: Fig. 110. Hydra fusca.]
These singular and voracious creatures increase like plants by budding.
A little protuberance rises on the body by the bulging out of the double
skin or wall, so that the interior of the bud is a clear cavity in
communication with the stomach of the Hydra (fig. 110, _b_). The bud
increases in length, opens at its extremity into a mouth, and gradually
acquires the size and form of its parent (fig. 110, _c_); the
communication is then by degrees closed, and at last the matured bud
drops off and becomes an independent Hydra. Dr. Carpenter observed that
this process, which so closely resembles the budding of plants, must be
regarded as a modification of the ordinary nutritious process. The same
may be said of the power of reparation, which every animal body
possesses in a greater or less degree, but which is most remarkable
among the lower tribes, for when an entire member is renewed, or even
when the whole body is regenerated from a small fragment, which is the
case in many polypes, it is by a process exactly analogous to that which
takes place in the reparation of the simplest wound in our own bodies,
and which is but a modification of the process that is constantly
renewing, more or less rapidly, every portion of our frame.
There is but one species of the single colourless Hydra, but there are
four compound fresh-water Hydræ in England—the rubra, viridis, vulgaris,
which is of an orange brown, and the fusca. They have coloured
particles, either imbedded in their external coat, or immediately under
it. The Hydra viridis and H. vulgaris have short tentacles, whilst H.
fusca, which is a rare animal, has arms from seven to eight inches long,
and so contractile, that they can shrink into the space of small
tubercules. All these four Hydræ are compound and permanently
arborescent animals; each springs from one individual hydra of its own
race, which increases in length and forms the stem, while young ones
spring from it and from one another consecutively, like the compound
branches of a tree. The numerous tentacles that hang down like fishing
lines, thickly covered with thread-cells and their envenomed darts,
catch prey for the whole colony, because the communication between the
stomachs of the young polypes or Hydræ and that of their parent is never
cut off, as it is when the offspring is deciduous; but tubes from the
base of each individual Hydra or polype, passing through the stalks and
branches of the living tree, unite their stomachs with the stomach or
assimilating cavity in the main stem. Each individual polype, sometimes
to the number of nineteen, after having digested its food or prey,
ejects the refuse from its mouth, and the nutritious juice traverses the
labyrinth of tubes to that general reservoir.
Since every portion of the bodies of the Hydræ is nearly of the same
kind, and as every part of their surface inside and outside is in
contact with the water in which they live, and from whence they derive
oxygen to aërate their juices, no circulation is necessary in these
simple animals, either for nutrition of their tissues, or to furnish
them with oxygen.
If the Hydræ only produced deciduous buds which are developed into
facsimiles of their parent, their race would become extinct, since they
die in winter, unless kept artificially in water of mild temperature;
but the animals are hermaphrodite, so that each individual produces
fertilized eggs in autumn, which are hatched in spring, so that the
Hydra is alternately propagated by deciduous buds and by eggs. The
fresh-water hydræ are the only hydroids that are locomotive, all the
others being fixed to some solid substance.
The oceanic Hydrozoa comprehend the three families of Corynidæ,
Tubulariidæ, and Sertulariidæ. They are chiefly compound animals,
numerous in genera and species, and have great variety of form. They may
be simple and slender, they may be creeping or like a bush or tree, more
or less compound and regularly branched according to the form of the
polypary or tubular substance which unites their numerous hydra-form
polypes into one animal. In general they are exceedingly small; three or
four inches in height is quite gigantic. There is scarcely a still clear
pool left by the retiring tide among the rocks along the British coasts,
that does not abound with these beautiful creatures attached to stones,
old shells, or sea-weeds. But they must be sought for amidst the
luxuriant marine vegetation and profusion of animal life which adorn
these rocky pools, otherwise they would escape notice; and even when
large enough to be conspicuous, the eye must be aided in order to see
the wonderful minuteness and delicacy of their structure. The aquaria
have furnished an opportunity to study their forms, habits, and the
marvellous circumstances of their lives and reproduction.
The compound oceanic Hydrozoa are essentially the same in structure as
the compound fresh-water Hydræ. They differ, however, from them in often
having a greater number of tentacles, and in being defended by a firm
and flexible horny coat; notwithstanding which they increase in size by
budding from the base of a single primary polype. The horny coat covers
the bud and grows with it; but as soon as the polype is formed within
it, the top of the bud opens and the young polype protrudes itself, so
that a separation is effectually prevented; and while the stem and
branches are being formed, and increase by the continual development of
new buds, the communication between the stomachs of the whole brood of
polypes with that in the parent stem is maintained by tubes from their
bases passing through the interior fleshy matter in the branches.
In short these marine Hydrozoa consist of a ramified tube of sensitive
animal matter, covered by an external flexible and often jointed and
horny coat or skeleton, and they are fed by the activity of the
tentacles and the digestive powers of frequently some hundreds of
hydra-formed polypes, as in the Sertularia cupressina. The common
produce of their food circulates as a fluid through the tubular
cavities, for the benefit of the whole community, while the indigestible
part is ejected from the mouth of each individual. The stomach of each
polype has a more or less ciliated lining, containing cells with
nutritive juices, which are supposed to perform the part of a liver. The
liquid which circulates in these animals is colourless, with solid
particles floating in it; and there is reason to believe that sea-water
is admitted into the tubes, and that, mixed with the juices prepared by
the polypes, it circulates through the ramified cavities, is sent into
the hollow prehensile tentacles, and returns back into the digesting
cavity after having contributed to respiration by its oxygen. The
movements of this fluid appear to depend upon the delicate ciliated
fibre which lines the cavities of the tentacles and those of the stem
and branches of the compound animal, possibly aided by vital
contraction. The soft skin of the tentacles contains cells full of
liquid, with a thread and its sting or dart coiled up within it. These
thread stings are protruded when the skin is irritated, which frequently
gives the tentacles the appearance of being beset with bristled warts.
In many instances these kinds of Hydrozoa are covered with a gelatinous
substance, either as a film or thick coat.
The reproduction of many of these arborescent or compound Hydrozoa is
one of the most unexpected and extraordinary phenomena in the
life-history of the animal creation. For besides the system of
consecutive budding from a single polype which builds up the compound
animal, peculiar buds are formed and developed, which bear no
resemblance whatever to the polype buds: on the contrary, when mature,
they assume an organization exactly the same as that of the common
jelly-fish or Medusoid Acalephæ, and swim freely away from their fixed
parent as soon as they are detached. These medusiform zooids, which are
extremely small, consist of a cup or umbrella-shaped bell of colourless
transparent matter, which is their swimming apparatus; it is contracted
and expanded by a muscular band under the rim, the water is alternately
imbibed and forcibly ejected, and by its reaction the zooid is impelled
in a contrary direction. From the centre of the bell a stomach hangs
down in the form of a proboscis, with a mouth at its extremity, either
with or without tentacles and sting-cells. Four canals, or a greater
number, which begin in the stomach, radiate through the transparent
matter of the bell, and are united by a circular canal round the rim;
they convey the nutritious liquid from the stomach throughout the
system. This general structure may be traced in the zooids of the three
great families of the oceanic hydraform-zoophytes, in a greater or less
degree, from deciduous perfect medusæ to such as are imperfect and
fixed.
These medusiform zooids are male and female, and when detached from
their parent they are independent creatures, each of them being
furnished with nutrient and locomotive organs of its own. They produce
fertilized eggs, which are developed into ciliated locomotive larvæ;
after a time these lose their cilia and acquire a rayed sucking disc,
with which they fix themselves permanently to a solid object, and, after
various changes, each gets a mouth and tentacles and becomes a perfect
young hydra. Thus a brood of young hydræ is produced, each of which
acquires the compound form of its parent by budding, and as each of
these compound animals in its turn gives off medusa-buds, there is a
cycle of the alternate forms of hydra and medusa or jelly-fish, showing
a singular connection between two animals which seem to have nothing in
common. The analogy which so often prevails between plants and animals
obtains here also, for the medusa-buds bear the same relation to the
hydra or polype-buds that the flower-buds of a tree do to the leaf-buds:
the flower-buds contain the germs of future generations of the tree,
while the leaf-buds contain only the undeveloped stems, stalks, and
leaves of the individual plant on which they grow.
The Corynidæ form the first of the three families of the oceanic hydra
zoophytes. They comprise six genera, and many species of compound
animals of various forms, each derived from a single animal by budding;
and although they possess a thin flexible coat, the polypes are sheathed
either in a thin membrane or bone. Their club-shaped tentacles form
either a single or double circlet round the base of their conical mouth,
and are also scattered over their bodies when bare.
[Illustration: Fig. 111. Syncoryna Sarsii with Medusa-buds.]
The zooids are developed at once in the Syncoryna Sarsii, which is a
long, thinly branched, and horny zoophyte, with a single naked,
spindle-shaped polype at the extremity of each branch, as in fig. 111,
A. The bodies of the polypes are studded with numerous tentacles, among
which buds appear (fig. 111, _a_, _b_); these gradually expand into
bell-shaped medusa-zooids (fig. 111, _c_), some being masculine and
others feminine. They drop off their parent, swim away by the
contraction of their bell, and their fertilized eggs are developed into
single hydræ, which become arborescent like their parent by budding.
The family of the Sertulariidæ take branching forms, sometimes of
perfect symmetry: they have a firm, horny coat, which not only covers
the stem and branches, but becomes a cup for the protection of the
polype. The most common form of the family of the Tubularia has no
branches: it has an erect, hollow stem like a straw, sometimes a foot
high, coated by a horny sheath. The polype which terminates each plant
has a mouth surrounded by alternately long and short tentacles. The
stomach of the polype is connected with the hollow in the stem by a
muscular ring, by whose alternate dilatation and contraction, at
intervals of eighty seconds, the fluid is forced up from below, enters
the stomach, and is again expelled. Another liquid carrying solid
particles circulates in a spiral through the whole length of the stem.
Some of this family are propagated by perfect deciduous medusæ, others
by imperfect fixed ones; both are developed on the polypes or among
their tentacles. Like the fresh-water Hydræ, these creatures can restore
any part of their bodies that is injured.
Numerous instances might be given to show that the minute medusiform
zooids are only a stage or phase in the life of an oceanic hydra:
conversely it will now be shown, that the single simple hydra is but a
stage in the life-history of the highly organized medusa, jelly-fish, or
sea-nettle of sailors, the Acalepha of Cuvier.
The medusæ vary in size, from microscopic specks that swim on the
surface of the sea in a warm summer day to large umbrella-shaped jelly
fish almost a yard in diameter. They abound in every part of the ocean
and in all seas, often in such shoals that the surface of the water is
like a sheet of jelly. Their substance is transparent, pure, and nearly
colourless; chiefly consisting of water, with so little solid matter,
that a newly caught medusa, weighing two pounds, dries into a film
scarcely weighing thirty grains.
The Pulmograde Medusæ, which swim by the contractions of their
umbrella-shaped respiratory disc, form two distinct groups, the
naked-eyed medusæ and the covered-eyed group. Both are male and female;
each has its own form of thread-cells; and the stinging power or
strength of the poison is nearly in proportion to the size of the animal
and the coarseness of its threads.
The disk, or umbrella-shaped swimming organ, in both groups consists of
a large cavity included between two layers of gelatinous matter, which
unite at the rim. The interior membrane, called the sub-umbrella, is
encircled at its edge by a ring of highly contractile muscular fibre
like the iris of our eyes, by which this swimming organ is expanded and
contracted. From the centre of the sub-umbrella a stomach, in the form
of a proboscis, is suspended, which is of a very different structure in
the two groups.
[Illustration: Fig. 112. Thaumantia pilosella.]
The Thaumantia pilosella, a member of the naked-eyed group, is like an
inverted watch-glass (fig. 112), less than an inch in diameter. The roof
of this umbrella is much thicker than the sides, and gradually thins off
towards the rim. The proboscis, or stomach, descends from the centre of
the sub-umbrella, but not so far as to the edge of the rim: it ends in a
mouth with four sensitive fleshy lips. Four slender canals, which
originate in the cavity of the stomach, radiate from the centre of the
roof of the umbrella and extend to its margin, where they unite at the
quadrants with a canal which encircles the rim, and are prolonged beyond
it in the form of tentacles armed with numerous thread-cells containing
poisonous darts. These tentacles must be formed of muscular fibre, for
they are very irritable: each of them may be extended and contracted
separately or along with the others; they guide the medusa through the
water, and can anchor it by twisting round a fixed object.
The prey caught is digested in the stomach, the refuse is ejected by the
mouth, and the nutritious fluid that has been extracted is carried up
through the base of the stomach into the four radiating canals, to
supply the waste and nourish the system. The digestive cavity and canals
are lined with a soft membrane, covered with cilia, whose vibrations
maintain the circulation of the juices and perform the duty of a heart;
for the medusæ have none, nor have they any special respiratory system:
their juices are aërated through the under-surface of the rim of the
umbrella, while passing through the circular canal lying either within
the water or on its surface.
A fringe of filamental tentacles hangs down into the water from the rim
of the disc or umbrella, which is studded at equal distances by fleshy
bulbs, each of which has a group of fifty dark eye-specks, being the
rudiment of an eye; and if the animal be disturbed when in the dark,
each eye-speck shines with a brilliant phosphoric light, and the
umbrella looks as if it were begirt with a garland of stars.
[Illustration: Fig. 113. Otolites of Magnified Thaumantias.]
Close to the edge of the canal which encircles the margin of the
umbrella, there are eight hollow semi-oval enlargements of the flesh,
two in each quadrant formed by the four radiating canals: they are the
eight ears of the medusa, for in these hollow organs there are from
thirty to fifty solid, transparent, and highly refractive spheres,
arranged in a double row, so as to form a crescent, those near its
centre being larger than the more remote. The solid spheres are
analogous to the otolites in the ears of the more highly organized
animals. Mr. M‘Cready has discovered nerve-centres behind each tentacle,
and under each marginal coloured speck in several species of the
open-eyed medusæ, which places this group of Acalephæ in a higher grade
than any of the preceding orders. The medusæ swim by the muscular energy
of their umbrellas: at each rhythmical contraction the water, which
enters by the mouth and fills the great central cavity within the
umbrella, is forced out again through an orifice at the other end, and
by its reaction the medusa is impelled with considerable velocity in the
contrary direction, so that the top of the umbrella goes first, and all
its tentacles are dragged after it.
The medusæ are diœcious: in the males four reproductive cells full of
reddish or purple granular matter surround the cavity of the stomach,
and appear like a coloured cross through the top of the gelatinous
umbrella. In the females, at a point just before the four radiating
canals enter the marginal canal, the flesh on the exterior of the
umbrella swells out into bulbs, containing vessels full of clear eggs
with minute globular yolks. These eggs, when fertilized, are hatched,
and the young are developed within these ovaries, so that they come into
the water as a kind of infusorial ciliated animalcule destitute of a
mouth. One end of the creature acquires a suctorial disc, fixes itself
to an object, and uses its cilia. The other end opens into a mouth,
round which tentacles like fishing lines spring forth; the central part
is converted into the cavity of the stomach, and thus a perfect hydra is
formed, capable of being propagated naturally by budding, or
artificially by being cut in pieces, each piece becoming a perfect
hydra, differing in no respect from a common simple fresh-water Hydra.
[Illustration: Fig. 114. A, B, C, D, development of Medusa-buds; _a_,
polype-body; _b_, tentacles; _c_, a secondary circle of tentacles; _d_,
proboscis; _e_, new polype-bud.]
From one of these, numberless successive generations of simple hydræ may
be produced by budding, all catching their prey with their tentacles and
digesting it in their stomachs. The limits to this budding-system seems
to be indefinite: years may pass in this stage, but at length it ceases,
and either the original hydra, or one of its descendants, undergoes a
series of remarkable changes. The body of the hydra lengthens into a
cylinder; it is then marked transversely by a number of constrictions
beginning at the free end; these become deeper and deeper, till at
length they break up the body into a pile of shallow cups, each lying in
the hollow of the other, and leaving a kind of fleshy wall at the point
of suspension or fixture. The edges of the cups are divided into lobes
with a slit in each, in which the coloured rudiment of the eye is sunk.
The cups are permanent, and characteristic of the group of naked-eyed
medusæ. After a time, the cups begin to show contractile motions, which
increase till the fibre of their attachment is broken, and then the
superimposed cups are detached from the pile one after another, and swim
freely away by the contractions of their lobes as young medusæ, leaving
what remains of the parent hydra to repair its loss and again repeat
this singular process. However, the young medusæ are not yet perfect. As
they increase in size the divisions on the edge of the cup fill up; a
proboscis-shaped stomach, with its four coloured cells and its square
mouth, is developed from the centre of the sub-umbrella; the radiating
canals extend from the central cavity, the encircling canal and fringe
form round the umbrella-shaped cups, and the result is a highly
organized Thaumantia pilosella, in whose life-history a simple hydra
forms a singular stage.
Thus hydræ produce medusæ whose offspring are hydræ, and perfect medusæ
produce hydræ whose offspring are perfect medusæ. However, the law of
the alternation of generation is by no means peculiar to the Thaumantiæ.
Many species of medusæ are subject to it, as the Turris neglecta, a
beautiful little medusa not larger than a hempseed, common on the
British coasts. It has a white muscular pellucid umbrella, a large
proboscis of a rich orange colour at its upper part: in the
orange-coloured flesh of it there are ovaries containing rose-coloured
eggs, which are hatched within them, and come into the water as ciliated
gemmules, which, after swimming about for a time, become fixed and are
developed into small hydræ of a rich purple colour with sixty-four
tentacles. From these hydræ others bud off indefinitely till the time
comes when one of them becomes lengthened, constricted, divided into
cups which drop off, and finally become a brood of the Turris neglecta.
The naked-eyed medusæ are extremely numerous. There are six orders of
them and many genera, chiefly distinguished by the position and nature
of their ovaries and the number of canals which radiate through their
swimming organs. Both of the medusæ that have been described have four
radiating canals; yet they belong to different orders, for the ovaries
of the Thaumantia are in the edge of the umbrella, while those of the
Turris are in the substance of the proboscis. Neither of these kinds
have more than four ovaries, but some other kinds have eight ovaries and
eight radiating canals. Most of the canals are simple, but in one genus
they are branching. All are furnished with tentacles, some of them
having stings, others none.
The covered-eyed group consists only of two natural divisions—the
Rhizostoma, or many-mouthed medusæ, and the Monostoma, or one-mouthed
medusæ. In both the coloured eye-specks at the margin of the umbrella
are larger and more numerous, than in the naked-eyed group, and they are
covered with a hood. The proboscis of the one-mouthed order terminates
in a square mouth, the four angles of which are prolonged into tentacles
with a solid hyaline axis. They have a fringed membrane along their
under-surface, containing numerous stinging thread-cells. Sixteen
canals, connected with the stomach or cavity of the proboscis, radiate
over the flattish, cup-shaped umbrella; eight of these are branched, and
terminate in the circular canal which runs round its fringed edge, and
they form the nutrient and respiratory system of the animal, while the
eight simple and alternate canals terminate in eight openings at the rim
of the umbrella, through which the refuse or indigestible part of the
food is discharged, thus forming an exception to the other pulmograde
medusæ, and indeed to the Hydrozoa in general, which eject it at the
mouth. All the canals are lined with cilia, whose vibrations maintain
the circulation of the fluids, and perform the duties both of a heart
and respiratory apparatus. Dr. A. Krohn has observed that in three
species of the genus Pelagia belonging to the covered-eyed medusæ, the
young are at once developed as medusæ without the intervention of the
hydra form.
[Illustration: Fig. 115. Rhizostoma.]
The disk of the Rhizostoma, or root-mouthed medusæ, is rather flat, and
the large proboscis is unlike any other of the tribe. In the naked-eyed
medusæ digestion is performed in the cavity of the proboscis; but in
this order the proboscis is divided into four very long branches ending
in club-shaped knobs (fig. 115), and nutrient tubes extend to their
extremities from the great central cavity in the umbrella. Their
broadish frilled borders are divided and subdivided along their whole
lengths, and the nutrient canals, which follow all their ramifications,
end in numerous fringed pores upon their edges and upon the club-shaped
ends of the quadrifid proboscis. These numerous pores are mouths; they
absorb minute animalcules, which are digested while passing through the
united canals to the great central cavity of the umbrella, which
receives the products of digestion. Eight canals radiate from that great
cavity and traverse the umbrella; and the nutrient fluid, mixed with the
sea-water, passes from the great cavity through these canals into an
elegant network of large capillary tubes spread on the under-surface of
the margin of the umbrella, which is always in contact with the water;
and in this beautiful respiratory organ the carbonic acid gas is
exchanged for the oxygen in the water of the sea. The indigestible part
of the food is discharged through the mouths or pores, whose edges are
prolonged into solid tentacles containing thread-cells, with their usual
weapons of offence and defence. Besides these armed tentacles, which are
very numerous in the covered-eyed group, the gelatinous umbrella has a
multitude of oval thread-cells on its external coat, in each of which a
very long filament is spirally coiled, which darts out to a considerable
distance on the smallest touch, and stings severely.
A few only of the British pulmonigrade medusæ sting: the Cyanea
capillata, one of the single-mouthed covered-eyed family, is most
formidable. It has very long tentacles, which it can throw off if they
get entangled, but they continue to sting, even after they are detached
from the medusa.
This is one of the most remarkable instances of the inherent
irritability of muscular fibre still in full force after the tentacles
have been separated from the living animal. In many of the lower
animals, as in the Hydra itself, vitality is so far from being
extinguished in the severed members that it repairs the injury. Since
the covered-eyed medusæ have eyes, ears, and very sensitive tentacles,
it may be inferred that they possess nerves of sight, hearing, and
touch, though none have been discovered, probably on account of the
softness and transparency of their tissues. The stinging power by which
they kill their prey and defend themselves may be classed among the
consensual powers prompted by the sympathetic sensations of hunger or
danger.
In all latitudes the medusæ are highly luminous, especially in warm
seas. Professor Vogt remarked that flashes of light passed over their
disk when they touched one another in swimming, and they appear at
intervals like globes of fire among the lesser lights of the Noctilucæ;
if from involuntary nervous contraction, as is most likely, the light
must be electric.
The medusæ are infested by many parasites. Entozoa are often abundant in
their gelatinous substance, and crustaceans of various kinds and
colours, such as shrimps, sand-hoppers, and a galæmon of glassy
transparency, move about in the substance of their disc and arms,
entering unscathed by the poisonous darts which inflict instant death on
others of their class. The Libanea crab, of gigantic size compared with
its host, is in the habit of taking up its abode between the four
columns of the Rhizostoma. But the most singular intruder is the
Philomedusa Vogtii, which is a polype with twelve thick short tentacles,
its whole body and tentacles being covered with cilia and thread-cells.
These polypes live in the disk, arms, and stomach of the medusæ, and,
when taken out, their stomachs are found to contain fragments of the
tentacles of their host, and even the thread-cells with their stings.
The larger polypes devour the smaller ones, and the latter live for
weeks within the larger ones without apparent inconvenience to
either.[24]
Mr. M‘Cready mentions that the larvæ of the medusa Cunina octonaria swim
as parasites in the cavity of the bell of the medusa Turritopsis
nutricula, which not only furnishes a shelter and dwelling-place to the
larvæ during their development, but it also serves as a nurse, by
permitting the parasites, which adhere by their tentacles, to take the
food out of its mouth by means of their long proboscides. They undergo
many transformations, and become nearly perfect medusæ while within
their nurse.
Medusæ of different species are met with in every sea from the equator
to the poles. They are eminently social, migrating in enormous shoals to
great distances. The largest shoal of young sea nettles on record was
met with in the Gulf Stream, off the coast of Florida, by a vessel bound
for England. The captain likened them to acorns; they were so crowded as
completely to cover the sea, giving it the appearance from a distance of
a boundless meadow in the yellow leaf. He was five or six days in
sailing through them, and in about sixty days afterwards, on returning
from England, he fell in with the same school, as the sailors call it,
off the Western Islands, and was three or four days in sailing through
them again. Mr. Piazzi Smyth, when on a voyage to Teneriffe in 1856,
fell in with a vast shoal of medusæ. With a microscope he found part of
the stomach of one of these creatures so full of diatoms of various
forms—stars, crosses, semicircles, embossed circles and spirals—that he
computed the whole stomach could not have contained less than 700,000.
The flinty shells of the diatoms ejected in myriads by the medusæ,
accumulate in the course of ages into siliceous strata, which, heaved up
by subterranean fires, at length become the abode of man. Thus
gelatinous transparent beings indirectly aid in forming the solid crust
of the earth by means of the microscopic vegetation of the sea.
_Ciliograde Hydrozoa._
The ciliograde Acalephæ, which form four orders and many genera, and
which swim by means of symmetrical rows of long cilia, are represented
on the British coasts by the Cydippe pileus and the Beroë Forskalia
(fig. 116), little delicately tinted, gelatinous, and transparent
animals that shine in the dark.
The Cydippe pileus is a globe three-eighths of an inch in diameter, like
the purest crystal, with eight bands of large cilia, stretching at
regular distances from pole to pole. A mouth, surrounded by extremely
sensitive tentacles, is situated at one pole, the vent at the other. The
Cydippes poise and fix themselves to objects by means of two very long
tentacles, fringed on one edge by cirri, that is, short curled
tentacles. These cirrated tentacles, which in some species stretch out
to more than twenty times the length of the animal, can be
instantaneously retracted into cavities at the posterior end of the
body, while, at the same time, the marginal filaments are as rapidly
coiled up in a series of close spirals. The whole of these complex
organs are enclosed within the limits of a pin’s head.
[Illustration: Fig. 116. A, Cydippe pileus; B, Beroë Forskalia.]
The manner in which these little gems swim is beautiful; sometimes they
rise and descend slowly, like a balloon, and when they glide along the
surface of the water in sunshine, the cilia on the eight meridional
bands exhibit the most brilliant iridescence. The long cirrated
tentacles follow all their motions in graceful curves, or hang
indolently down, and sometimes they are suddenly stretched to their full
length, and as suddenly retracted, and in all their varied convolutions
the cirri that fringe them are in constant vibration, and exhibit all
the tints of the rainbow. Sometimes these creatures whirl round their
axis with great rapidity, but, active as they are, no nervous system has
yet been discovered in them.
_Fig. 117, p. 103._
[Illustration: PRAYA DIPHYS.]
The common Beroë is like an elongated melon, obtusely octangular, with
eight rows of cilia, extending from a mouth at one end to a kind of
ciliated star at the other. The Beroës are of a gelatinous transparent
substance, which expands and contracts with great facility: it is always
expanded when they swim.
The Cestum Veneris belongs to another genus of the same family. It is
like a blue ribbon, the mouth and vent being on the opposite sides in
the middle of the band, which is furnished throughout its whole length
with active cilia for swimming. The ciliograde Hydrozoa are monœcious,
and do not produce medusa-zoids.
_Campanograde Acalephæ._
There is a group of oceanic Hydrozoa, consisting of several families,
which are fed by numerous suctorial organs called polypites, with
tentacula and thread-cells attached to their bodies, so that they are
analogous to the marine hydræ, in being colonies of individuals united
into a compound animal. Some have air-vessels, which enable them to
float on the surface of the water; but the locomotive organs of this
group are bells, so that they may be called Campanograde Acalephæ.
The family of the Diphyidæ are colourless, and of such transparency that
they are all but invisible when in the water, and are gelatinous masses
clear as crystal when taken out of it. They are chiefly inhabitants of
the warmer parts of the Pacific and Atlantic Oceans, but many fine
specimens are found in the Mediterranean. Of these the Praya diphys is
one of the most extraordinary (fig. 117). It has two large
swimming-bells, their mouths turned backwards, with which the whole
community is connected. They are nearly equal in size, soft, gelatinous,
transparent, and colourless, rounded in front, open and truncated
behind. The adjacent sides are parallel, with a groove between them,
into which one end of the long tubular filiform body of the animal is
fixed by slender tubes, through which a nourishing liquid passes into
radiating canals in the bells, and from them into a circular canal at
their margins, which are surrounded by a muscular contractile iris, like
that in our eyes, which shuts and opens the bells. By the alternate
absorption and ejection of the water the bells go head foremost, and
regulate the motions of the whole compound animal. When both bells are
active it goes straight forward; when the right hand bell is alone in
action, it goes to the left, and _vice versâ_; in fact, the bells act as
a rudder.
The slender cylindrical body or axis of the Praya is so transparent,
that the cavity and muscular fibres of its walls are distinctly seen.
These animals are extremely contractile. Professor Vogt mentions an
individual he met with at Nice more than three feet long, when extended
on the surface of the water, which could contract itself into little
more than a finger length. It was said to have had a hundred isolated
groups of polypites with their appendages attached to it; but in general
the Prayæ are not so long, and seldom have more than thirty or forty of
these isolated groups, which are attached to the under-side of the long
flexible body, and hang down like a rich and beautiful fringe. In the
figure, the position of the numerous groups of polypites and their
appendages are merely indicated by round marks and lines.
In the body of the Praya diphys (fig. 117), as in that of the whole
family, there is a nutritious liquid, which, by means of cilia, flows on
its interior surface in two directions: it enters the canals in the two
large bells, and supplies them with nourishment.
The polypites which digest the food are vermiform double sacs
communicating at one end by a valve with the canal in the body of the
animal; and at the free end they are prolonged into a mouth with an
everted lip, and the digesting apparatus lies in the centre. Each
polypite is supplied with food by its own fishing-line descending from a
point close to where the polypite is fixed to the long axis. It is a
long, tubular, branched tentacle, each branch ending in a coloured,
pear-shaped, or fusiform battery of thread-cells with their stings. A
gelatinous plate is placed on the upper side of the common axis
immediately over the isolated groups, to protect and separate them.
Such are some of the most general characters of the family Diphyidæ: the
Praya diphys has something peculiar to itself.
In the Praya, each individual group has a swimming-bell of its own
adjacent to the polypite, and lying parallel to the axis of the animal,
with its mouth turned backwards. It is connected by tubes both with the
general central canal, and with a helmet-shaped protecting plate. On the
other side of the polypite, there is a tuft of vermiform buds with
spiral terminations, bristled with thread-cells. From the centre of this
tuft a tentacle, or fishing-line, descends with numerous branches, the
whole forming a tubular system connected with the common canal in the
axis. Each of the branches of the tentacle terminates in a
vermilion-coloured tendril, coiled up into a minute capsule. The inside
of the tendril is not only bristled with the points of sabre-shaped
darts, but it conceals a filament crowded with thread-cells. On the
slightest touch, the tendril stretches out like a corkscrew of red
coral, and every dart springs forth. Such is, more or less, the
complicated structure of the offensive and defensive weapons of many of
this order of oceanic Hydrozoa, which appear to the naked eye as merely
brightly-coloured points. The use of these tentacles, or fishing-lines,
is the same in all; they seize, kill, and carry their victims to the
mouth of the polypite by contracting their long lines.
In the Praya, the groups are individualized in the highest degree
consistent with union; for, when the animal is at rest, each of the
individual groups, amounting to thirty or forty, swims about by means of
its little bell independent of the rest. Their motions can be compared
to nothing but a troop of jugglers performing gymnastic exercises round
a cord represented by the common body of the animal; except for
adherence to which the life and will of each group are so perfectly
independent, that the mutual dependence of the whole is only seen when
the common trunk contracts to bring all its appendages towards the two
principal bells, which then begin to move.[25]
Thus each group has a special life and motion, controlled by a general
life and motion; strong individual muscular power controlled by general
muscular power; yet no nervous system has as yet been discovered, so
this animal activity must for the present be attributed to a strong,
inherent, contractile power in the muscular fibre. The Praya is seldom
complete, on account of the ease with which it casts off its great
bells.
None of the Diphyidæ have special organs for respiration; their juices
are aërated through their delicate tissues. They are diœcious, and
invariably produce perfect male and female medusiform zooids; they are
situated among the groups of the polypites and their appendages, and are
attached to the axis of the animal. When free, they swim away by the
contraction of their bells; the eggs are fertilized, and produce young
Diphyidæ, male and female; so these animals, like most of the oceanic
Hydrozoa, have two alternate stages of existence.
_Physograde Acalephæ._
The Galeolaria lutea (fig. 118, frontispiece) is similar to the Praya
diphys in having a slender, tubular body, with groups of sterile
polypites and their appendages hanging at intervals along its under-side
like a fringe, and also in having two swimming-bells at its anterior
extremity; but it has no special small bells. The large ones differ from
each other in size, form, and position. The largest is nearly
cylindrical, its mouth is turned upwards, and its rim is elevated at one
part into two stiff organs like the blinkers that are put over horses’
eyes: besides these, it has six salient points, which nearly close the
mouth of the bell at each contraction of the muscular iris that lines
the margin of the cavity. The small bell, which goes first in swimming,
is thicker and shorter, and its side rises in a hump, upon which the
closed end of the large bell rests, and in the cavity between the two
the anterior extremity of the filiform body is fixed. Each of the groups
of polypites, with their tentacles, lies immediately under its
spathe-shaped protecting plate. The polypites are very contractile, and
on their protuberant part, containing the digestive cavity, there is a
large circular space, which, as well as the whole tissue of the polypite
and the stinging capsules at the extremities of the tentacles, are of an
orange colour, and are akin in structure to those described.
When very young the Galeolaria and its congeners have only their
swimming-bells and one polypite group affixed to the end of a short
tubular axis; by and by a second group is developed from buds between
the bells and the first group; then a third is developed between the
bells and the second group, and so on; the length of the body and the
number of groups continue to increase indefinitely. It is only when the
animal is full grown and complete in all its parts, that reproductive
organs are developed towards its posterior end. Buds then appear upon
the hollow stems of the polypites towards the posterior end of the body.
But as the Galeolaria is diœcious, male and female buds are never on the
same individual. The female buds become medusiform zooids, like those of
the Praya diphys, only the transparent cup, with which it swims away
from its parent, has two projections like ears on its rim.
The development of the buds in the male Galeolaria is similar. At first
they are pale, but they assume an orange red colour as they advance
towards maturity, and, when complete, the sac which hangs down from the
centre of the transparent cup becomes of a brilliant vermilion. These
male and female medusa-zooids swim about for several days, and the
fertilized eggs are hatched into young Galeolariæ, male and female.
Thus the Galeolaria lutea has two kinds of polypites, both nutritive,
but one is sterile, the other prolific. The latter are similar to the
prolific individuals of the syncorine Hydræ, in which the anterior part
is a digestive organ, while on the base or stalk true medusa-zooids are
found. It is curious that the spathe-protecting plate of the Galeolaria
appears in the egg as a globe of such size that the other parts seem to
be merely the appendages.
_Fig. 119, p. 108._
[Illustration: APOLEMIA CONTORTA.]
The Apolemia contorta (fig. 119) unites the most graceful form to the
utmost transparency and delicacy of tissue. It has a double float, the
first small and globular, the second long and oval. The neck is short,
the rose-coloured body is flat as a ribbon, and covered with thin,
curved, pointed, and imbricated plates, like tiles on the roof of a
house, but so minute that they are only perceptible to the naked eye by
a slight iridescence. At the extremity of the short neck buds,
semi-developed buds, and perfect swimming cups are arranged in vertical
series; and as the flat body is twisted into a spiral to its farthest
end, the cluster of bells forms a perfect cone with the float at its
apex. The bells are flattened; and there is always in their more solid
posterior part a single canal rising directly from the general trunk
which divides into four branches; and these, having traversed the
swimming cavity, unite anew in a circular canal, or iris, destined to
shut and open the cup.
_Fig. 120, p. 109._
[Illustration: PHYSOPHORA HYDROSTATICA.]
The sterile polypites that are attached at intervals by their hollow
stems to the twisted body of the Apolemia, have twelve rows of cells
inserted in the bright lining of their digestive cavities; their
anterior part has a trumpet-shaped mouth full of thread-cells. The
tentacles affixed to their stems and their secondary lines are like
those of the Diphyidæ. Besides these sterile polypites, which serve only
to feed the animal, the Apolemia has a kind of mouthless prolific
organs, which do not contribute to the general nourishment: each group
has a pair of them attached to the extremity of a branching stem. They
resemble polypites in being long and contractile at their extremities;
the interior is full of a substance like sarcode, and encompassed by a
red ring. Female buds yielding eggs appear on the stem of one of these
organs, while male buds are developed into medusiform zooids on the stem
of the other, which become detached, swim away, and the fertilized eggs
yield young Apolemiæ.
The natural position of an individual of the family of the Physophoridæ
when at rest is to hang perpendicularly from its air-vessel. The body,
which begins with a pyriform float, descends in a slender filiform
scarlet tube with a number of hyaline natatory cups or bells attached on
each side. The lower end of the body enlarges into a bulb or disk
supporting various appendages.
The Physophora hydrostatica (fig. 120), common in the Mediterranean, has
a transparent pear-shaped air-vessel tipped with red, from which the
slender cord-like tube of the body descends. Immediately below the
air-vessel, a number of buds and young bells are attached, followed by a
series of perfect three-lobed swimming bells, placed on each side
obliquely one below the other; and as they alternate and embrace the
body with their deeply excavated sides, they give it the appearance of a
crystal cone. Four canals spring from the hollow stalks of the bells,
traverse them, and end in a circular canal close to the membranaceous
iris which surrounds the margin of the internal cavity. Below the cone
the tubular body expands, and is twisted into a flat spiral, so as to
form a hollow disk or bulb, to which three different circlets of organs
are appended. The first and uppermost is a coronet of red, worm-like,
closed sacs, in constant motion, with large thread-cells at their
pointed extremities. They are attached to the upper surface of the bulb
by their broad bases, and communicate with its tubes by a small valve.
Male and female capsules follow either in a circle, or mixed with the
third and undermost circlet of organs, which consist of sterile
nourishing polypites, fixed by hollow stems to the bulb, each of which
has a long branching tentacle fixed to the base of its digesting cavity.
There are as many polypites on the under-side of the bulb as there are
red worm-like sacs on its upper edge. Each polypite consists of three
distinct parts. The posterior part is a hollow red stalk inserted under
the circumference of the disk; the second part is a bright yellow
globular expansion containing the digestive cavity lined with cilia; the
third and anterior part, which ends with the mouth, is quite colourless
and transparent, and assumes various shapes by constant expansion and
contraction.
At the limit between the red stalk and the yellow globular part of the
polypites there is a tuft of cylindrical appendages, from which a long
tentacle descends with its secondary tentacles and red nettle-bulbs. All
the canals of this Physophora are connected, and their walls are lined
with muscular fibres, either circular, longitudinal, or both, which give
a marvellous contractile and motive power. When the animal is suspended
from the surface of the sea by its float, every member is in motion,
especially the numerous tentacles, which are perpetually in search of
food, and are so extremely sensitive that even a sudden motion of the
water makes them shrink under the red worm-like organs on the edge of
the disk. This animal is generally from one to three inches long.
All the preceding members of the physograde group are really
campanograde, for the action of the wind upon the floats of the
Physophoridæ must be small, otherwise they would not be furnished with
so many swimming cups. The Physaliidæ and Velellidæ are the only two
orders that are truly physograde, for the wind is their only locomotive
power.
The Physalia, or ‘Spanish man-of-war’ of sailors, is by far the most
formidable animal of the Acalephæ tribe; its poisonous stings, which
burn like fire, inflict instant death on the inferior animals, and give
painful wounds to man himself. Its body, as it floats, is a long
horizontal double sac (fig. 121), which begins with a blunt point,
gradually enlarges, and becomes cylindrical about the middle; then it
somewhat suddenly widens in a transverse or lateral direction. Along the
upper surface of the pointed half the membrane or wall of the sac is
raised into a transversely placed crest, which dies away at the enlarged
end. The greater part of the body is smooth, but the under-surface of
the transversely enlarged end swells into lobes, from whence numerous
tentacles and other organs descend.
Almost the whole of the body of the Physalia is filled by an air-vessel,
so that it floats on the surface of the sea, and is wafted to and fro by
the wind. The bladder containing the air is enclosed in two membranes,
the outer one dense, thick, and elastic, the inner formed of delicate
fibres and lined with cilia. The air-sac is only attached to one part of
the interior; and there it communicates with the exterior by a small
aperture, which may be seen at about half an inch from the pointed apex
of the animal. The body is several inches long, of a delicate pale green
colour, passing gradually into dark indigo blue on the under-surface;
the ridge of the crest is tipped with dark crimson, and the pointed end
is stained with deep bluish green.
[Illustration: Fig. 121. The Physalia.]
The appendages, which hang down from the inferior and thick part of the
body, are large and small branchless tentacles of various lengths, and
sterile polypites in different stages of development. In some
individuals the tentacles are nine or ten feet long, of a deep blue
colour at their origin, and formed of two distinct parts, which have a
common base. One is a long conical bag, formed by an extension of the
under-surface of the body lined with cilia, and ending in a pointed apex
full of stinging thread-cells. It is flat on one side, attached
throughout its length to the tentacle, and is supposed to furnish poison
for the stings. The tentacle itself is a closed tube whose canal
communicates with the cavity of the long sac, and consequently with that
in the animal’s body. The interior of the tentacle is ciliated, its
upper part is gathered into folds; and the rest, which hangs straight
down, is like a delicate narrow ribbon, highly contractile from muscular
fibres, of which the most conspicuous are longitudinal. The tentacle is
marked at regular intervals by blue kidney-shaped masses, containing
myriads of powerful thread-cells, in which the threads of the darts are
coiled in a spiral, and contain muscular fibres, that serve to contract
and extend them. The smaller tentacles vary as much in length as the
large ones; they are of similar structure, but of a paler colour, and
are indiscriminately mixed with the other appendages.
The polypites, which are direct processes from the under-surface of the
body, are crowded in groups of various sizes round the base of the large
tentacles and mixed with the small; they are of a deep blue at their
base, frequently of a bright yellow at their extremities, and on an
average about three-fourths of an inch long. They are as irritable and
contractile as the tentacles, and are in constant motion. Their mouth is
large, with an everted lip armed with thread-cells; it sucks in the prey
caught and brought to it by the contraction of the tentacles, and which
is speedily dissolved by the powerful solvent juices in its digesting
chamber.
Among the numerous appendages attached to the under-surface of the
Physalia, there are bluish-green velvety masses fixed to extremely small
branching processes from the body of the animal, which seem with a
microscope to consist of tentacles, polypites in various stages of
development, male reproductive capsules which are never detached, and
female buds that are developed into medusiform zooids, and are presumed
to become free as in other cases. The Physaliidæ are social animals,
assembling in numerous shoals in the warm latitudes of the Atlantic and
Pacific. They naturally have their crest vertical, kept steady by their
tentacles, which drag down in the water; but Professor Huxley has seen
them at play, in a dead calm, tumbling over and over. The Physalia does
not possess the power of emptying and refilling its float with air: it
is doubted whether any of the physograde animals have that power, but
the subject is still in abeyance.
The Velellidæ are little sailing members of the physograde group. The
Velella spirans (fig. 122), a Mediterranean species, has a body or deck
consisting of a hollow horizontal disk, of a firm but flexible
cartilaginous substance, surrounded by a delicate membranous fringe or
limb half the width of the body. A triangular vertical crest, formed of
a firm transparent plate, also encompassed by a delicate limb, is fixed
diagonally from one angle of the disk to the other, but not on the
fringe; and as the natural position of the Velella is to float
horizontally on the surface of the water, the crest is exposed to the
wind and acts as a sail.
The float or air-vessel is flat, horizontal, and nearly fills the whole
body of the animal: it consists of two thin, firm, and rather concave
plates joined at their free edges, and united also by a number of
concentric vertical partitions, between which there is a series of
concentric chambers or galleries filled with air. The chambers
communicate with one another by apertures in the dividing membrane; they
also communicate with the exterior by perforations through the surface
of the body. Very long pneumatic filaments, that is tubes filled with
air, descend from the inferior surface of the float, and pass through
the lower plate of the disk into the water.
[Illustration: Fig. 122. Velella spirans:—1, upper side; 2, under-side.]
The disk is transparent, and appears to be white from the air within it;
and it is marked by concentric rings corresponding to the divisions in
the air-vessel below. The fringe-like limb that surrounds it is flat,
flexible, semi-transparent, and of the richest dark blue passing into
green, with a light blue ring; it is very contractile, and moves in slow
undulations. The sail or crest is thin, firm, and transparent, covered
by a bluish membrane; its limb is dark blue, crossed by waving yellow
lines.
An irregular microscopic network of vascular canals, containing yellow
matter, is seen in the soft substance which covers the sail; it ends in
a canal round its margin. A similar system exists both in the upper and
under-surface of the disk. All these systems are connected with one
another, and with organs pending from the inferior side of the disk,
which are hid when the Velella is in its natural horizontal position.
These organs consist of a large central sterile polypite, which supplies
the whole system with elaborated juices; it is surrounded by smaller
polypites, which are both nutritive and reproductive; and the whole is
encircled with a ring of prehensile and armed tentacles fastened to the
rim of the disk, immediately adjoining to the under-side of the limb.
The pneumatic filaments already mentioned are mixed with these different
organs.
In the Velellidæ caught by M. Vogt, he invariably found the stomachs of
the large as well as of the small polypites, full of the carapaces of
minute crustacea, shells, the bones of small fishes, and larvæ, so as
even to be swelled out with them. The indigestible parts are thrown out
at the mouth, and the elaborated juices are transferred to the various
systems of canals to be distributed through all the members of the
animal. The mouths of the small polypites take various forms; sometimes
they are wide and trumpet-shaped, with everted lips, sometimes they are
contracted. These small polypites consist of a double sac, fastened to
the disk by a hollow stem with many rounded elevations on their surface
full of thread-cells. The tentacles of the Velellidæ are strong, thick,
club-shaped tubes, completely closed at their extremities, which abound
in thread-cells; their cavity is filled with a transparent liquid,
supposed to play an important part in their elongation.
Medusiform zooids are formed on the slender stems of the small
polypites. It is presumed that they lay fertilized eggs which yield
Velellidæ, so that this animal has probably alternate states of
existence; but nothing is known of its earliest stages of development.
The youngest form yet discovered is that described by Prof. Huxley, in
his excellent monograph on ‘Oceanic Hydrozoa.’[26] The Velellidæ are
inhabitants of warm and tropical seas, but are occasionally found on the
coasts of Great Britain, being carried by the Gulf Stream to the Bay of
Biscay, and thence wafted northwards by the prevalent winds.
Although the Porpita, a genus of the Velellidæ, has no sail, it is akin
to the Velellæ in size and structure. The body of the Porpita consists
of two circular cartilaginous disks, united at their edges and
surrounded by a blue membranous limb. On the surface of the upper disk
there are beautifully radiating striæ, each of which ends at the
circumference of the disk in a little protuberance, which gives it the
appearance of a toothed wheel. A large sterile polypite occupies the
centre of the under-surface of the body, surrounded by a zone full of
smaller ones; and the space between the zone and the blue limb is
occupied by a narrow area of a reticulated appearance, to which numerous
circles of tentacles are fixed, that spread out and radiate all around
the margin of the animal. The interior circular rows are simple, short,
and fleshy, not extending much beyond the edge of the limb: the
succeeding circles are gradually longer, while the exterior row, which
extends far beyond the limb, are branched and beset with slender
filaments, ending in minute globes, sometimes filled with air, so that a
Porpita is like a floating daisy, though differently coloured. The
Porpita glandifera, a pretty little inhabitant of the Mediterranean,
which only appears in calm weather, is not more than eight lines in
diameter; somewhat convex, white, marked by radiating striæ, and
encompassed by a dark blue limb. The central polypite and those next to
it are whitish, the rest become of a darker blue towards the limb; the
tentacles are pellucid and bluish, and the three last rows have little
dark blue globes attached to them by slender filaments.
The Porpita has a horizontal air-vessel divided vertically into
air-chambers like the Velella, but they are much more numerous. In a
middle-sized Porpita, four or five lines in diameter, there are
twenty-three or twenty-four air-chambers surrounding a central one, and
eighty or ninety pneumatic filaments, so that the animal is extremely
buoyant. Brown matter, supposed to be a liver, lies directly below the
undermost wall of the air-vessel, through which, as well as through the
base of the animal, all the pneumatic filaments penetrate; the greater
number go straight down into the water, but a portion of them terminate
in the walls of the polypites.
A complete system of canals, ciliated internally, traverses all parts of
the animal; and it may be presumed that the cilia maintain its juices in
a state of circulation similar to that in the Velella; and the functions
of the polypites, great and small, that are in connection with the
liver, are also similar to those of the Velella. The Porpita is armed
with thread-cells like all the class. The central polypite is sterile
and nutritive; the small ones are both nutritive and reproductive: buds
spring from their stems, which become independent male and female
medusiform zooids, swim away from their parent and produce abundance of
eggs, whence a new generation of Porpitæ arise.
In this singular class of fresh-water and oceanic Hydrozoa, the internal
cilia, aided by the contraction of the walls of the body, are the sole
means provided by nature for the circulation of the fluids.